LONDON, England (CNN) -- Britain has condemned Russia's refusal to extradite Andrei Lugovoi to face murder charges for the poisoning death of an ex-KGB agent in London as "extremely disappointing."

Andrei Lugovoy, above, is the main suspect in the murder of Russian emigre Alexander Litvinenko.

Prime Minister Gordon Brown's spokesman said Tuesday an offer of a trial in Russia had been ruled out by prosecutors because it would fail to meet standards of "impartiality and fairness."

The spokesman said Britain was now considering further options.

Russian authorities earlier confirmed Lugovoi would not be sent to Britain to face charges of killing Alexander Litvinenko.

Brown's spokesman said: "Russia's refusal to extradite Mr Lugovoy is extremely disappointing and we deeply regret that Russia has failed to show the necessary level of cooperation in this matter."

Russia has a constitutional ban on handing over its citizens to face criminal charges in another country.

Litvinenko -- a fierce critic of Russian President Vladimir Putin -- died in November, several weeks after being exposed to toxic levels of polonium-210.

Traces of the radioactive material were found at Litvinenko's London home and several places where he ate and met with others before becoming sick. Trace amounts were also found on several British and Russian airliners.

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"We have consistently said that the murder of Mr. Litvinenko is a serious criminal matter; hundreds of British citizens and visitors to the capital were put at risk," a UK Foreign Office representative said.

Last month, British officials announced they had enough evidence to charge Lugovoi -- a former security service agent-turned businessman -- with Litvinenko's murder.

Russian Foreign Minister Sergei Lavrov responded by accusing London of trying to turn the criminal case into a "political campaign."

Lugovoi met Litvinenko in London on November 1, 2006, at the Millennium Hotel bar, hours before the 43-year-old fell ill. Litvinenko released a deathbed statement blaming Putin for involvement in his poisoning -- an allegation the Kremlin denied.

Lugovoi has argued his innocence, and told reporters in Moscow last month that he thinks the British government may have been behind the agent's murder.

"It is difficult to abandon the idea that Litvinenko had become an agent of the British special services who got out of hand and was then eliminated -- if not by the special services themselves, then under their control or their connivance," he said.

The case has put pressure on relations between London and Moscow, and the recent developments have threatened to aggravate the situation further.E-mail to a friend